Process for treatment of molten stainless steel



Patented June 6, 1950 PROCESS FOR TREATMENT OF MOLTEN STAINLESS STEELHarry Tanczyn, Baltimore, Md., assignor, by mesne assignments, to ArmcoSteel Corpora tion, a corporation of Ohio No Drawing. Application July11, 1945, Serial No. 604,541

7 Claims. (Cl. 22-215) 1 This invention is a companion to that of mycopending application, Serial No. 604,542, filed July 11, 1945, andentitled Stainless steel process and product and relates to stainlesssteel, more particularly to a method of casting the steel and to acomposition for combatting stainless steel teeming and casting scums.

An object of my invention is the provision of a simple, reliable andhighly satisfactory method of producing stainless steel ingots or othercast products involving the teeming of metal into a mold or the like.

It is a further object to provide a, rapid, emcient and dependablemethod of producing stainless steel ingots of improved clean quality inthe cast condition.

A further object of my invention is the pro- .vision of an efficaciousaddition agent for combatting stainless steel teeming scums, which agentis thoroughly satisfactory to the result of rapidly fluxing the scumfrom the teemed steel.

A still further object of my invention is the provision of stainlesssteel ingots or the like which in the cast condition are of good surfacequality and substantially free of teeming scum inclusions.

Other objects in part will be obvious and in part pointed outhereinafter.

The invention accordingly consists in the combination of elements,composition of ingredients and mixture of materials, and in the severaloperational steps and in the relation of each of the same to one or moreof the others as described herein, the scope of the application of whichis indicated in the following claims.

As conducive to a clearer understanding of certain features of myinvention, it may be noted at this point that stainless steels aredefined as steels which comprise 10% to 35% chromium, with or withoutnickel, and with or without supplemental additions of columbium,titanium, aluminum, molybdenum, tungsten, vanadium, cobalt, copper,manganese, silicon, sulphur, and the like, for special purposes, and aremainder which is substantially all iron. The carbon content usually islow, this being on the order of 0.03% to 0.20% although it may be higherfor certain purpose steels.

A preponderant tonnage of stainless steel manufactured today issubjected at one time or another to teeming and casting operations suchas in the production of ingots from an initial melt or subsequent meltof the steel. In teeming the stainless steel into molds, a problem ofconsiderable magnitude arises which has to do with the prevention orelimination of undesirable scums in the cast metal. These scums usuallyare rich in oxides both of iron and chromium and on occasions include inquantity oxides of special purpose constituents of the steel. Likewise,nitrides of certain elements of the alloy steel usually contribute as afurther scum component along with the oxides. At times, the scums assumea refractory or semi-refractory quality which is thick, viscous, frothy,gummy or even semi-solid, and do not readily melt in contact with themolten stainless steel in the mold.

Such stainless steels as higher chromium steels, and stainless steelscomprising one or more of aluminum, titanium, columbium, and the like,give particularly refractory scums which are even more troublesome inthe mold than those of certain other types of corrosion resistant steel.In some instances of pouring stainless steel under conditions ofexposure to the atmosphere, the scum mounts up to several inches deepatop the cast metal. V

The fluidification or alleviation of the deleterious scums of teemingand casting stainless steel heretofore has been attempted with variousfluxing agents only with quite limited success and with markedineffectiveness in the instances of high chromium steels and steelsincluding any such elements as titanium, columbium, aluminum, or thelike.

The stainless steel teeming scums usually are driven into the newly caststeel by the force of the pouring stream or are turned under at theedges of the mold or are otherwise forced to inclusion in the cast metalduring the course of the teeming and casting operation. As a result thecastings after solidification require an extensive amount of surfaceconditioning such as grinding to remove non-metallics and metal ofinferior quality, or are rejected, which in either event is costly. Insome cases the included scums offer an obstacle to successful forging orrolling, or render the metal unsuitable for those uses where highquality Or strength is important. The cast stainless steels whichcontain segregations of scum therefore are susceptible to a host ofmetallurgical failures. They are not successfully polished to uniformbrightness and luster. In local surface areas the metal is subject toundue corrosion.

An object of my invention accordingly is the provision of a simple,direct and practical method of teeming and casting stainless steel,which method is highly eflicient in preventing or eliminatingdeleterious scums of teeming with respect to any of a wide variety ofstainless steels, even the higher chromium stainless steels and thosesteels including one or more such elements as titanium, columbium,aluminum, or the like.

Referring now more particularly to the practice of my invention, Iachieve in teeming and casting molten stainless steel, illustrativelyinto ingots, a highly effective prevention or elimination of deleteriousscums in the mold by use of one or more lithium salts of the groupconsisting of lithium fluoride, carbonate, oxalate, nitrate andchloride, in contact with the steel. In accordance with my invention, Iadd to the molten stainless steel a scum-fluxing agent comprising inpredominating amount or preferably consisting in substantial entirety ofone or more of the above noted salts of lithium, for example, to a ladleof the steel before lip pouring into a mold, or, preferably to the steelin teeming the same to the mold, or at the mold as by shifting thelithium salt inside the mold over the rising steel.

Of the salts just identified I find that the fluoride, carbonate andoxalate of lithium are particularly eflicient, alone, or together withone or more of each other, even in the casting of high temperaturestainless steels such as those comprising 20% or more chromium, andstainless steels including at least one of such elements as titanium,aluminum, columbium, or some other element or elements having a tendencyto produce a highly refractory non-metallic scum in and on the metalduring the casting operations whether the chromium content of the steelbe high or low. The fluoride of lithium is in general the most active ofall the lithium salts for defeating the deleterious scums of teeming andcasting the stainless steels and, therefore, I prefer to use this salteither in whole or in part.

By treating the molten stainless steel with one or more lithium saltsthe resulting stainless steel ingots or other cast products which Iachieve are of a. highly desirable quality substantially free of teemingscum inclusions. Likewise, the stainless steel castings have a muchsuperior surface and are remarkably devoid of scabs, coldshuts,wrinkles, rolls, splash marks, and like defects. The stainless steelingots which result from my pouring and casting method are satisfactoryfor reduction as by rolling with a minimum of preliminary conditioningof the surface thereof. The ingots give a high not working yield withlittle splitting or checking, as along forged or rolled edges, of thetype ordinarily casued by scum or other surface flaws, and give productswhich otherwise are substantially free of the efiects of teeming scum.

As illustrative of the practice of my invention I provide, as in asuitable electric furnace, a bath of 18-8 chromium-nickel stainlesssteel of 0.5% titanium grade. From the furnace I tap the metal into aladle preparatory to teeming the same into ingot molds, illustratvely ofinch by 10 inch average dimension inside on the horizontal. In preparingthe molds themselves, I put inside on the bottom of each a smallquantity, say for example approximately 2 ounces to 4 ounces, of acombatting scum agent consisting substantially all of lithium fluoride,the salt preferably being enclosed in small paper bags or the like.Thereafter I teem the stainless steel from the ladle into the molds andover the lithium salt therein.

During the teeming and casting operation the lithium fluoride in theingot molds thoroughly mixes with the molten steel and sets up a highlyeffective fluxing action in preventing and eliminating thick scums inthe mold. Should it be desired. additions of lithium fluoride are madeto the mold as by sprinkling to the cast metal as the teeming continues,or by mixing into the cast molten metal even after teeming, tosupplement or serve in lieu of previous additions of the salt to themold. I leave the scum-fluxed ingots to cool and solidify, andthereafter strip off the molds. The resulting chromium-nickel-titaniumstainless steel ingots are of good surface quality and are substantiallyfree of scum inclusions such as those including refractory nitrides oftitanium or oxides of chromium. As stripped, the ingots illustrativelyare ready for storage or sale, or are easily made ready for reductionand subsequently are reduced to such products as those rolled or forgedof the steel.

Thus it will be seen that there has been provided in this invention amethod and teeming scum fluxing agent, and products resulting, ofcasting stainless steel in which the various objects hereinbefore setforth together with many thoroughly practical advantages aresuccessfully achieved. It will be seen that the method is simple,effective and suitable for use in the production of castings from any ofa wide variety of stainless steels, and is industrially practicable withknown and tried casting apparatus.

As many possible embodiments may be made of my invention and as manychanges may be made in the embodiments hereinbefore set forth, it is tobe understood that all matter described herein is to be interpreted asillustrative, and not as a limitation.

I claim as my invention:

1. In the production in an iron metal mold of cast stainless steelproducts substantially devoid of scabs, cold shuts, wrinkles, rolls andsplash marks, the art which includes contacting molten steel duringteeming, and the scum formed by contact of the molten steel with theatmosphere, with a teeming scum flux essentially consisting of lithiumsalt of the group consisting of fluoride, carbonate, oxalate, chloride,and nitrate of lithium.

2. In the production in a mold of cast stainless steel productssubstantially devoid of scabs, cold shuts, Wrinkles, rolls and splashmarks in which the stainless steel contains at least one of titanium,columbium and aluminum, the art which includes adding to the moltensteel during the teeming thereof, and to the scum there formed, ateeming scum flux consisting of at least one lithium salt of the groupconsisting of the fiuo ride, carbonate, oxalate, chloride and nitrate oflithium.

3. In the production in a mold of cast stainless steel productssubstantially devoid of scabs, cold shuts, wrinkles, rolls and splashmarks in which the steel contains at least 20% chromium, the art whichincludes adding to the molten stream of metal during teeming, and to thescum there formed, a teeming scum flux consisting of at least onelithium salt of the group consisting of the fluoride, carbonate,oxalate, chloride and nitrate of lithium.

4. In the production of stainless steel ingots substantially devoid ofscabs, cold shuts, wrinkles, rolls and splash marks, the art whichincludes teeming metal into an ingot mold, and during said teemingsprinkling on the surface of, the rising metal a teeming scum fluxessentially consisting of lithium salt of the group consisting of thefluoride, carbonate, oxalate, chloride and nitrate of lithium.

5. In the production of stainless steel ingots substantially devoid ofscabs, cold shuts, wrinkles, rolls and splash marks, the art whichincludes placing in an ingot mold a teeming scum flux essentiallyconsisting of lithium salt of the group consisting of the fluoride,carbonate, oxalate, chloride and nitrate of lithium, and then teemingmetal into the mold.

6. In the teeming of stainless steel in an iron metal mold, the artwhich includes contacting the molten steel in the mold and the scumthere formed with a teeming scum flux consisting of at least one lithiumsalt of the group consisting of fluoride, carbonate, oxalate, chlorideand nitrate of lithium.

7. In the teeming of stainless steel in an iron metal mold, the artwhich includes adding to the mold a teeming scum flux essentiallyconsisting of lithium fluoride, and then teeming the metal thereinto.

HARRY TANCZYN.

REFERENCES crrEn The following references are or record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,672,446 Cape June 5, 19281,748,217 Grossman Feb. 25, 1930 2,265,985 Allen Dec. 16, 1941 FOREIGNPATENTS Number Country Date 110,022 Austria Mar. 28, 1940 423,489 GreatBritain Feb. 1, 1935 476,115 Great Britain Dec. 2, 1937 509,442 GreatBritain July 17, 1939 OTHER REFERENCES Chemical Rubber Publishing 00.,Cleveland, Ohio.

Refining Metals Electrically, Page 140; edited by Barton. Published in1926 by the Penton Pub- 20 lishing 00., Cleveland, Ohio.

1. IN THE PRODUCTION IN AN IRON METAL MOLD OF CAST STAINLESS STEELPRODUCTS SUBSTANTIALLY DEVOID OF SCABS, COLD SHUTS, WRINKLES, ROLLS ANDSPLASH MARKS, THE ART WHICH INCLUDES CONTACTING MOLTEN STEEL DURINGTEEMING, AND THE SCUM FORMED BY CONTACT OF THE MOLTEN STEEL WITH THEATMOSPHERE, WITH A TEEMING SCUM FLUX ESSENTIALLY CONSISTING OF LITHIUMSALT OF THE GROUP CONSISTING OF FLUORIDE, CARBONATE, OXALATE, CHLORIDE,AND NITRATE OF LITHIUM.